Multistate vibronic coupling between the excited 2Π states of the NO molecule

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Abstract
The fine‐structure analyses of highly resolved low‐temperature absorption spectra of four different isotopes of the NO molecule in the vacuum ultraviolet by Miescher and co‐workers have revealed a wealth of detailed information, among other results, on the 2Π states of the npπ Rydberg series leading to the first ionization limit and on their interactions with the vibrational progressions of the excited valence states B 2Π and L 2Π. However, measured spectral lines remain unassigned and spectroscopic results on the higher members of the B and L progressions and on their interactions with the npπ Rydberg series are incomplete. The present work attempts a quantitative representation of the observations in terms of five electronic states with mutual electronic coupling, i.e., a so‐called diabatic basis. The electronic states: two valence states B and L, and the lowest three Rydberg states of the npπ series C, K, and Q are defined by RKR potential curves, by spin‐orbit coupling constants, by electronic absorption dipole transition moments, and by six R‐independent electronic valence‐Rydberg interaction energies. The coupling is treated with a vibronic interaction matrix which includes the bound vibrational levels of these five electronic states up to the dissociation limit of the B 2Π state, i.e., a total of 69 vibrational levels in each of the two 2Π spin doublets and in each of three different isotopes. The calculated results include vibronic energies, B values, absorption oscillator strengths, and, for a few selected levels, detailed rotational structures. Assignments of v in the vibrational progression of the B state to observed structures in the spectrum have been extended up to v = 37, i.e., to within 226±30 cm−1 of the dissociation limit. The electronic valence‐Rydberg interaction energies are found to decrease along the Rydberg series approximately proportional to n*−3/2. The quantitative relationship between the spectroscopically determined two‐state interaction energies and the corresponding calculated matrix elements is clarified. Assignments of observed features to a vibrational progression of a valence state L 2Πi with a large spin‐orbit coupling constant are confirmed and the positions of unobserved spin components are predicted.